A.T. Robertson Commentary John 6:62

"[What] then if ye should behold the Son of man ascending where he was before?" — John 6:62 (ASV)

What then if ye should behold (εαν ουν θεωρητε). No "what" in the Greek. Condition of third class with εαν and present active subjunctive, "if ye then behold."

Ascending (αναβαινοντα). Present active participle picturing the process.

Where he was before (οπου ην το προτερον). Neuter articular adjective as adverb (accusative of general reference, at the former time as in 9:8; Galatians 3:13). Clear statement of Christ's pre-existence in his own words as in 3:13; 17:5 (cf. 1:1-18).